In this study, the performance and efficiency of CO2 huff-and-puff process for improving oil recovery and subsequent storage of CO2 in fractured porous media is examined and the results of laboratory tests are presented. The experimental set up consisted of a high-pressure stainless steel cell made specially to hold a cylindrical core with spacing around it to simulate fractures surrounding matrix. The matrix was saturated with normal decane, which was used as oil during these experiments. Over six sets of huff-and-puff experiments, using CO2 as solvent, were conducted for pressures of 250, 500, 750, 1000, 1250, and 1500 psi. Each set of the huff-and-puff experiments were conducted by injecting CO2 in the fracture surrounding the core (injection step). Then, the system was shut-in for a period of 24 hours to allow CO2 to diffuse from fracture into the oil in matrix (soaking period step). At the end of soaking period, the pressure was released and the oil production was measured (production step). The above cycle was repeated until no more oil was produced. The results obtained showed that CO2 huff-and-puff process improves the oil production from fractured media, significantly. These results also indicate that the oil recovery is higher for huff-and-puff experiments conducted at higher pressures.
Introduction
Application of Huff-and-Puff process has been tested as a means of implementing variety of enhanced oil recovery processes, such as CO2 and hydrocarbon solvent injection in conventional oil reservoirs. Huff-and-puff process improves oil recovery through oil swelling, hydrocarbon extraction, viscosity reduction and relative permeability effects (Monger and Coma, 1988, Haskin and Alston, 1989). Since such a technique is applicable through single well, it is also recognized as relatively low initial capital outlay and rapid payout (Haines and Monger, 1990). Many studies on applicability of huff-and-puff process have been conducted through utilizing various miscible and immiscible solvents for improving oil recovery from oil reservoirs (Heines, 1990; Lino, 1994; Johnston, et al., 1990). While, sequential steam huff-and-puff technique can increase the oil recovery from heavy oil reservoirs significantly (Shuhong, et al., 2005), cyclic CO2 injection has been proposed as an alternative to cyclic steam stimulation for heavy oil reservoirs. However, it is found that CO2 huff-and-puff process has wider applications in light-oil reservoirs (Thomas and Monger, 1990). An early application of immiscible cyclic CO2 injection process was reported by (Gondiken, 1988) through a pilot project carried out by Turkish Petroleum Corporation in Camurla Field that has a heavy crude oil of 11–12 °API. The results of this early application of huff-and-puff immiscible CO2 project were not economical in all stages due to the lack of equipment and some field problems. However, significant reduction in the oil viscosity and interfacial tension resulted in improving oil recovery in some stages of this project. Also, there was an evidence of dissolution of calcium carbonate by carbonic acid, which resulted in improved permeability. Another study by Monger and Coma (1986) showed that post waterflood residual oil could be displaced by CO2 huff-and-puff process in a light crude oil Berea core. According to their studies, operating at minimum miscibility pressure (MMP) is not beneficial and process is not sensitive to duration of soak period. However, they suggested additional investigations on the effect of reservoir and operating pressure on the efficiency of huff-and-puff process. Later on, Haskin and Alston (1989) evaluated CO2 huff-and-puff tests in 12 fields in east and south of Texas. They confirmed that soak times of 2 to 3 weeks result the same oil recovery as longer soak periods. However, both laboratory study by Monger and Coma (1988) and limited field data analysis by Haskin and Alston (1986) show that oil recovery can be improved by increasing the amount of CO2 injected. During immiscible CO2 cyclic injection, it has been seen that ultimate oil recovery is largely dependent on oil saturation. Since solubility of CO2 is a function of pressure, at conditions below MMP a small amount of CO2 is dissolved in the oil. Hence, in order to have better ultimate oil recovery CO2 must contact a large amount of oil and initial oil saturation has significant impact on the efficiency of the process (Miller, 1990). Formation and precipitation of scale during immiscible CO2 injection is anther challenge encountered during CO2 huff-and-puff process. An evidence of such a problem was seen in Crooks Gap Field, Wyoming, during immiscible CO2 injection (Smith, et al., 1991). Dissolution of CO2 in the formation water results in formation of carbonic acid, which in turn can dissolve formation minerals during injection and soak period. Although this process improves formation permeability (Crawford, et al., 1963). However, if calcium carbonate precipitates near the wellbore, this will lead to reduced productivity (Mackay et al., 2003). Wang and Gaoming, (2006) claimed that in the watered-out horizontal wells, shut-in time and total gas injection volume have less impact on the ultimate recovery compare to injection rate and injection time during huff-and-puff process. The results of this parametric study is slightly in contrast with the experimental and field analysis results presented by Haskin and Alston (1989) and Monger and Coma (1988), which concluded that larger CO2 volumes recover more incremental oils. Denoyelle and Lemonnier, (1987) investigated the effect of gas relative permeability on efficiency of huff-and-puff process. They found that, gas relative permeability had significant impact on the recovery factor of CO2 huff-and-puff process. Results of different studies (Monger and Coma, 1988; Haskin and Alston, 1989; Mohammed-Singh, et al., 2006) show that reduction in oil viscosity and interfacial tensions, relative permeability alteration, oil swelling, vaporization of lighter components of oil by CO2 and in some cases removal of near well bore damage are the main mechanisms contributing to improve the oil recovery under CO2 huff-and-puff injection include.
